1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to a fringe field switching mode LCD having pixel electrodes and counter electrodes configured to produce a fringe field having horizontal components parallel to the surface of a back substrate thereof.
2. Description of the Related Art
An in-plane switching (IPS) mode LCD is well known, which has been developed by Hitachi, Ltd., Japan. Such an IPS mode LCD exhibits superior viewing angle characteristics over twisted nematic (TN) mode LCDs because it is driven based on a lateral field. However, the IPS mode LCD exhibits degraded characteristics in terms of the ratio of aperture and the transmittance of light. In order to improve the degraded characteristics of such an IPS mode LCD, a fringe field switching mode LCD has also been proposed.
The configuration of a conventional fringe field switching mode LCD is schematically illustrated in FIG. 1. FIG. 1 is a plan view of the conventional fringe field switching mode LCD.
As shown in FIG. 1, the conventional fringe field switching mode LCD includes a back substrate 1. Gate bus lines 2 and data bus lines 4 are arranged on the back substrate 1 in a matrix form to define unit pixels R, G, and B.
In the vicinity of an intersection where one gate bus line 2 and one data bus line 4 cross each other, a thin film transistor (TFT) is arranged, which serves to apply a signal voltage from the data bus line 4 to liquid crystal molecules in a liquid crystal cell or to cut off the application of the signal voltage.
In each unit pixel, a counter electrode 5 and a common electrode line 7 are formed. The counter electrode 5 has a rectangular plate shape, and a common electrode line 7 adapted to supply a common signal to the counter electrode 5. The counter electrode 5 is made of a transparent conductive material. The common electrode line 7 has a first electrode portion 7a extending in parallel to the gate bus lines 2, and a second electrode portion 7b extending from the first electrode portion 7a in parallel to the data bus line 4 while being arranged between the counter electrode 5 and the data bus line 4. The first electrode portion 7a is electrically in contact with the counter electrode 5 whereas the second electrode portion 7b is electrically insulated from the data bus line 4.
Also, in each unit pixel, a pixel electrode 9 is formed, which is made of a transparent conductive material. The pixel electrode 9 overlaps partially with the counter electrode 5 while being insulated from the counter electrode 5 by a gate insulating film not shown. The pixel electrode 9 has a plurality of uniformly-spaced comb portions 9a extending in parallel to the data bus lines 4, and a pair of bars 9b adapted to integrally connect the comb portions 9a together at opposite ends of each comb portion 9a, respectively.
Although not shown, the conventional fringe field switching mode LCD also includes a front substrate, a liquid crystal cell, a back polarization plate, and a front polarization plate. The front substrate faces the back substrate 1 in a state in which the liquid crystal cell is interposed between the front and back substrates. The back polarization plate is attached to the back substrate 1, and the front polarization plate is attached to the front substrate. The spacing between the front and back substrates is more than the spacing between the counter electrode 5 and the pixel electrode 9 in order to allow a fringe field to be generated in the liquid crystal cell. A back horizontal alignment film is formed between the back substrate 1 and the liquid crystal cell. The rubbing axis of the back horizontal alignment film defines an angle of xc2x112xc2x0 with respect to the gate bus lines 2.
Now, the operation of the conventional fringe field switching mode LCD having the above mentioned configuration will be described where a voltage is applied between the counter electrode 5 and the pixel electrode 9 in one unit pixel.
Between the counter electrode 5 and the pixel electrode 9, a fringe field is produced which extends to the entire portion over the counter electrode 5 and over the pixel electrode 9. By virtue of this fringe field, the liquid crystal molecules in the liquid crystal cell are activated, thereby the screen of LCD being in a white state.
Since all liquid crystal molecules in the liquid crystal cell arranged over the counter electrode 5 and pixel electrode 9 are activated, it is possible to obtain a high aperture ratio and a high transmittance.
Meanwhile, the transmittance of an LCD can be expressed by the following expression:
T≈T0 sin2(2xc3x97)xc2x7sin2(Πxc2x7xcex94nd/xcex)
where, T represents, the transmittance of the LCD, T0 represents the transmittance of the LCD for a reference light, X represents an angle defined between the optical axis of liquid crystal molecules and the polarization axis of polarization plate, xcex94n represents the refractive index anisotropy of the liquid crystal molecules, d represents the spacing between the front and back substrates, and xcex represents the wavelength of incident light.
Referring to the above expression, it can be found that a maximum transmittance is obtained when X equals Π/4 or xcex94nd/xcex equals xcex/2. The refractive index anisotropy An of the liquid crystal molecules depends on the viewing direction in which the screen of the LCD is viewed. When the refractive index anisotropy An varies, xcex94nd and xcex corresponding to a maximum transmittance are varied. For this reason, a color shift phenomenon occurs. That is, the color corresponding to the wavelength xcex varied depending on the viewing direction is displayed on the screen of the LCD.
For example, where the direction viewing the screen of the LCD corresponds to the short axis of the liquid crystal molecules, a decrease in An occurs, and the wavelength of incident light corresponding to a maximum transmittance is shortened. As a result, the viewer is rendered to view blue color with a wavelength shorter than that of white light.
On the other hand, where the viewing direction corresponds to the long axis of the liquid crystal molecules, an increase in An occurs, and the wavelength of incident light corresponding to a maximum transmittance is lengthened. Accordingly, the viewer is rendered to view yellow color with a wavelength longer than that of white light.
Such a color shift phenomenon occurs severely in the above mentioned fringe field switching mode LCD because the liquid crystal molecules in each unit pixel are oriented in the same direction in the white state.
Therefore, an object of the invention is to provide a fringe field switching mode LCD in which the pixel electrode and counter electrode of each unit pixel are formed such that a fringe field produced in one unit pixel has an orientation different from those of fringe fields respectively produced in adjacent unit pixels to solve the above mentioned problem involved in the related art,.
In accordance with the present invention, this object is accomplished by a fringe field switching mode liquid crystal display comprising a front substrate, a back substrate facing the front substrate, a liquid crystal cell interposed between the front and back substrates, a plurality of gate bus lines and a plurality of data bus lines arranged on the back substrate in a matrix form to define a plurality of unit pixels, a plurality of counter electrodes each formed at an associated one of the unit pixels and made of a transparent material, the counter electrodes to which desired common signal is applied, respectively, a plurality of pixel electrodes each formed in a region adjacent to an associated one of the counter electrodes to produce a fringe field having horizontal components in cooperation with the associated counter electrode, the pixel electrodes being made of a transparent material, a plurality of thin film transistors each connected to an associated one of the gate bus lines, an associated one of the data bus lines, and an associated one of the pixel electrodes and adapted to charge the associated pixel electrode based on an input signal from the associated gate bus line and an input signal from the associated data bus line, a front horizontal alignment film formed between the liquid crystal cell and the front substrate to have a rubbing axis of a desired direction, a back horizontal alignment film formed between the liquid crystal cell and the back substrate to have a rubbing axis of a desired direction, a back polarization plate formed at a back surface of the back substrate to have a polarization axis of desired direction, and a front polarization plate formed at a front surface of the front substrate to have a polarization axis extending in a desired direction, wherein the counter electrodes and the pixel electrodes are arranged on the back substrate such that the horizontal components of the fringe field produced at one of the unit pixels is orthogonal to the horizontal components of the fringe field produced at each of the unit pixels longitudinally and laterally adjacent to the one unit pixel.
And to improve response speed by increasing the rotating force of the liquid crystal molecules, the rubbing axis of the back horizontal alignment film defines an angle of xc2x145xc2x110xc2x0 with respect to each of the gate bus lines.
The above objects, and other features and advantages of the present invention will become more apparent after a reading of the following detailed description taken in conjunction with the accompanying drawings.